High gain, high-energy solid-state lasers that operate in the eye safe region (wavelength>1.3 micrometers) are in demand for military and commercial applications. These lasers, are typically based on crystals doped with erbium (Er) atoms that are pumped with semiconductor lasers at approximately 980 nanometers to achieve very high powers (power>3 watts). Semiconductor lasers using indium phosphide base material can also achieve laser light in this spectrum range, but at significantly lower powers (less than 1.5 Watts). There are several military requirements for such lasers. One is the augmentation of fire control systems with the capability to identify the target (Target ID) using laser radar (LADAR) imaging techniques. Another is the development of ultra-high power lasers for improved missile defense systems. Commercial applications of eye safe lasers include the development of free space communication nodes in conventional fiber optic networks and laser cutting/welding systems for manufacturing. Emerging medical applications require high-power multimode lasers in the spectral range from 1310 nm to 1600 nm. Within this wavelengths range, light is absorbed by targeted tissues for elimination of skin wrinkles, acne, and top-skin (for skin resurfacing). In the cases of acne and wrinkle reduction, the spectral band near the water absorption peak is ideally suited to absorb energy in the targeted areas and depths within the skin. This allows for direct destruction of p-acne and old collagen cells, prompting the body to build new, healthier skin tissue.
The Er-doped crystals have a number of absorption bands located between the visible and the near infrared spectral regions. Currently, such Er-based lasers are pumped by semiconductor diode sources operating at 0.98 micrometers (μm). Since the eye-safe lasers operate at 1.5 microns and are not 100% efficient, the difference in energy between pump beam and laser emission gives rise to heat within the laser medium. Consequently, thermal management becomes a critical issue in developing high power, eye-safe, solid-state lasers, particularly for applications that are sensitive to power dissipation, system footprint, and supporting facilities such as cooling and ventilation. Diode sources operating within the spectral ranges of either 1.47-1.48 or 1.53-1.54 microns would provide much more efficient pumping of the Er-doped crystals. This would lead to higher energy, higher gain operation with minimal energy loss to the host medium. The predominance of 0.98 micron diode sources in erbium-doped fiber optic amplifiers for telecommunication applications is due to the requirement that optical fiber amplifiers operate in single mode fiber. Many high power applications, however, are not constrained to single mode operation.
Conventional laser diode designs for infrared wavelengths, 1300 to 1600 nanometers (nm), use indium phosphide (InP) based materials with indium gallium arsenide phosphide (InGaAsP) doped quantum well structures. This structure is commonly used for high-frequency optical communications lasers that operate at low powers, less than 25 milliWatts (mW). Unfortunately, this common structure suffers from very poor efficiency and low output power, which are caused by poor electron and hole confinement, lower thermal conductivity of InP, and the difficulty of effective thermal management throughout the laser chip itself. The poor electron and hole confinement permits electron-carrier leakage around the quantum well structure, especially as the laser increases in optical power and resulting heat-load. This electron leakage converts directly into thermal energy without contributing to the optical output, and the additional thermal energy increases the chip temperature, which promotes further electron leakage. This cycle escalates very quickly such that only low optical powers are achieved before the laser reaches the thermal limit, whereby the optical energy decreases with additional electrical current. This limitation is called “thermal rollover”.
Conventional laser diode designs for telecommunications applications at infrared wavelengths are further typically optimized for low electrical power dissipation, low threshold current, and high modulation bandwidth along with a single output optical mode at relatively modest powers. This is because telecommunications applications require lasers with high gain and short optical cavity lengths. The requirement for high gain constrains the device design to a multiplicity of quantum wells, in the range of four to seven to reach the requirements of a low threshold current and sufficient optical gain for the short cavity length. The short cavity length also results in a higher active layer operating temperature for a given current. As a result the maximum output power before thermal runaway is constrained to low levels.